You notice an accident in the tunnel. What do you have to do?

In a tunnel, the main priorities are to warn following traffic quickly and to get professional help as fast as possible, because visibility is limited, escape routes are restricted, and a small incident can escalate (rear-end crash, fire, smoke).

Your choices 1 and 2 are correct:
Switching on the hazard warning lights immediately warns drivers behind you much earlier than brake lights and reduces the risk of further collisions in the confined tunnel space.
Using the SOS emergency telephone is the preferred way to call for help in tunnels because it connects you directly to the tunnel control center and gives them your exact location automatically. That allows them to trigger tunnel-specific measures (warning signs, speed reductions, closing lanes/tunnel tubes, ventilation, and directing rescue services).

Answer 3 (“Do not leave your vehicle”) is not generally correct. In many tunnel accidents you may need to leave the vehicle to reach a safe area, especially if there is smoke or fire, and follow the escape route signs to an emergency exit. As a rule of thumb: stay only if there is no immediate danger and you are safe; if there’s fire/smoke or you are told to evacuate, leave the car (key left in), move to the emergency walkway/exit, and follow instructions.